The present invention relates to the field of power electronics and, more particularly, to methods and systems for electric power quality protection.
Power quality plays a significant role in the modern aerospace/military industry. This is particularly true in the area of more electric architecture (MEA) for aircraft and spacecraft.
The commercial aircraft business is moving toward having electrical no-bleed-air environmental control systems (ECS), electrical variable-frequency (VF) power distribution systems, and electrical actuation. A typical example is the present Boeing 787 platform. Also, the Airbus A350 airplane will incorporate a large number of MEA elements. In the future, the next-generation Boeing airplane (replacement for the 737) and the next-generation Airbus airplane (replacement for the A320) will most likely use MEA. Some military aircraft already utilize MEA for primary and secondary flight control, among other functions.
Military ground vehicles have migrated toward hybrid electric technology, where the main propulsion is performed by electric drives. Therefore, a substantial demand for increased power electronics in these areas has emerged.
Future space vehicles will require electric-power-generation systems for thrust vector and flight control actuation. These systems must be more robust and offer greatly reduced operating costs and safety compared with many of the existing Space Shuttle power systems.
These new MEA aerospace and military trends have significantly increased the installed electrical sources and loads, along with the challenges to accommodate electrical equipment to new platforms. This has led to increased operating voltages and efforts to reduce system losses, weight, and volume. A new set of electrical power quality requirements has been created to satisfy system performance. Traditionally, the sources (electric generators) are required to maintain certain power quality requirements and their loads are to be able to operate at these requirements. Also, the loads are required to not create power quality disturbances on the distribution buses above certain levels. Yet the probability of power quality issues has increased due to the large number of installed electric equipment and their complex interactions.
Power quality is required to allow for compatibility between sources and loads installed on the same power distribution bus. A typical aircraft electric power system consists of a main power source, an emergency power source, power conversion equipment, control/protection equipment, and an interconnect network (i.e. wires, cables and connectors). The main power source comprises the main generators, driven by the aircraft propulsion engines. Emergency power is extracted from aircraft batteries, aircraft independent auxiliary power units (APUs), and aircraft ram air or hydraulically driven generators.
Power quality requirements for AC electrical equipment consist of a large number of parameters. Some of these parameters include current distortion, inrush current, voltage distortion, voltage modulation, power factor, phase balance and DC content. Current distortion, composed of AC harmonics, is the key design driver for electrical equipment. The requirements for current harmonics, subharmonics, and interharmonics specify the allowable distortion as a function of multiples of the fundamental frequency of the equipment input voltage.
A typical AC current harmonic includes all odd harmonics up to 39, with limits ranging from 10 to 0.25 percent of the maximum current fundamental. The current distortion requirement is a key design driver since it usually significantly impacts the equipment weight. Current distortion also is specified as a function of the equipment-rated power because higher power equipment has more influence on the power bus. Widely used specifications for power quality are MIL-STD-704-A to F, Airbus ABD0100, and Boeing TBD.
Power quality is a major concern for MEA aircraft because a large number of electric power systems and equipment are installed on the same electrical bus. The power quality of these systems and equipment has stringent requirements to ensure that all power supplies/utilization equipment function properly together.
For power supply equipment additional monitoring features are implemented to detect and isolate equipment, or groups of equipment, that may experience a power quality issue. This isolation capability is to protect other operating power supplies and utilization equipment. For power utilization equipment, strict power quality requirements are imposed. Some reasons for the requirements are as follows: (a) equipment contributing to power quality problems causes other equipment to fail; (b) equipment is prevented from achieving its design performance or reliability due to the reduced power quality of the source; (c) perhaps to meet a desired minimum weight, equipment designed with no power margin tends to be more susceptible to power quality issues; and (d) equipment designed to minimize weight tends to create power quality issues.
In the existing state-of-the-art power system, the power utilization equipment does not have power quality protection, or it is limited to over-voltage and under-voltage protections only. There are scenarios where a single equipment failure may propagate and create bus power quality non-compliance, leading to potential additional failures. For example, a single power source failure could fail to isolate power quality deficiencies and may damage utilization equipment or other power sources. A single utilization equipment failure may create non-compliant power quality bus and lead to other utilization equipment failure and/or power source failure. Utilization equipment can experience a destructive failure due to its own power quality non-compliance, and utilization equipment may fail regardless of the source of the power quality non-compliance on the bus.
As can be seen, there is a need for improved power quality protection of power utilization equipment.